1. Field of the Invention
The present invention relates generally to electrically active ceramic devices and, more particularly, to a method for making monolithic internally asymmetrically stress biased piezoelectric or electrostrictive devices having an integral electrode and the devices formed thereby.
2. Description of the Prior Art
Piezoelectric and electrostrictive materials develop a polarized electric field when placed under stress or strain. Conversely, they undergo dimensional changes in an applied electric field. The dimensional change (i.e., expansion or contraction) of a piezoelectric or electrostrictive material is a function of the applied electric field. Piezoelectric and electrostrictive materials can possess a large number of combined and useful properties such as piezoelectric (electric field dependent strain), electrostrictive, dielectric, pyroelectric (temperature dependent polarization), ferroelectric (electric field dependent polarization) and electrooptic (electric field dependent optical birefringence).
Recently, electrostrictive devices have generated considerable interest because of their increased strain under sizable loads as well as not requiring high voltages. In addition, modification of piezoelectric device geometries to increase achievable strain are also of interest. These devices have a wide range of applications which include actuators, pumps, speakers, sensors, switches, hydrophones, hydrospeakers, adaptive optics, variable focus mirrors and lenses, vibrators, benders, accelerometers, strain gauges and saddle inchworms.
Under an applied electric field, a piezoelectric crystal deforms along all its axes. It expands in some directions and contracts in others. The piezoelectric or strain coefficient describing this deformation is commonly denoted by the tensor d.sub.ij. EQU d.sub.ij =x.sub.j /E.sub.i (constant X)=P.sub.i /X.sub.j (constant E)
where x equals strain (extension per unit length); X equals stress (force per unit area); E equals electric field (volts per meter), and P equals polarization (Coulombs per square meter). The subscripts i, j refer to the crystal axes, or in the case of ceramics, to the direction of polarization of the ceramic. For example, d.sub.31 is the strain coefficient in the lateral direction while d.sub.33 is the strain coefficient for the longitudinal direction.
A typical ceramic device such as a direct mode actuator makes direct use of a change in the dimensions of the material, when activated, without amplification of the actual displacement. The direct mode actuator includes a piezoelectric or electrostrictive ceramic plate sandwiched between a pair of electrodes formed on its major surfaces. The device is generally formed of a material which has a sufficiently large piezoelectric and/or electrostrictive coefficient to produce the desired strain in the ceramic plate. By applying a voltage of appropriate amplitude and polarity between some dimensions of the device, it will cause the piezoelectric (or electrostrictive) material to contract or expand in that direction. When the device expands or contracts in one dimension (the thickness or longitudinal direction) it generally contracts or expands respectively, in dimensions in a plane perpendicular thereto (planar or transverse directions).
Direct mode actuators utilize either the longitudinal extensional mode or lateral extensional mode and are capable of sustaining high loads under compression (in excess of 1000 lbs. on a 3/4 inch rod under an applied electric field of 25 V/mil). However, direct mode actuators suffer from the disadvantage of a very small displacement (strain) that they are able to achieve which is at best a few tenths of a percent.
Indirect mode actuators achieve strain amplification via external structures. An example of an indirect mode actuator is a flextensional transducer. Flextensional transducers are composite structures composed of a piezoelectric ceramic element and a metallic shell, stressed plastic or fiberglass structure. The actuator movement of conventional flextensional devices commonly occurs as a result of expansion in the piezoelectric material which mechanically couples to an amplified contraction of the device in the transverse direction. In operation, they can exhibit up to about 0.5% strain at .+-.25 V/mil applied electric field and can sustain loads up to several hundred pounds.
Recently, a new type of flextensional transducer called a "moonie" has been developed. For example, U.S. Pat. No. 4,999,819 discloses a transducer which includes an electroded piezoelectric plate bonded to and sandwiched between two metal plates each having a concave cavity. Moonie's provide better performance and larger displacements than conventional flextensional transducers. For example, under an applied field of 25 V/mil, a two layer moonie with a total thickness of about 148 mils can exhibit a displacement of 1.5 mils (1.02% strain). However, as a result of its ability to sustain more strain than a conventional transducer, moonies can only sustain loads which are less than 100 lbs.
Other examples of indirect mode actuators include the unimorph, bimorph, multimorph and monomorph actuators. A typical unimorph is composed of a single piezoelectric element externally bonded to a flexible metal foil which is stimulated by the piezoelectric element when activated with a changing voltage and results in an axial buckling or deflection as it opposes the movement of the piezoelectric element. The actuator movement for a unimorph can be by contraction or expansion. Unimorphs can exhibit a strain of as high as 10% but can only sustain loads which are less than one pound.
A conventional bimorph device includes an intermediate flexible metal foil sandwiched between two piezoelectric elements bonded to the plate. Electrodes are bonded to each of the major surfaces of the ceramic elements and the metal foil is bonded to the inner two electrodes. A multilayer device known as a multimorph can be made by stacking alternating layers of ceramic elements and metal plates. When a voltage is applied to the electrodes, the bimorph or multimorph bends or vibrates. Bimorphs and multimorphs exhibit more displacement than unimorphs because under the applied voltage, one ceramic element will contract while the other expands. Bimorphs and multimorphs can exhibit strains up to 20% at 25 V/mil but as with unimorphs, cannot sustain loads greater than one pound.
A typical monomorph bender includes a piezoelectric plate with conductive electrodes disposed on each side thereof and is capable of bending similar to a bimorph. However, the bend in a monomorph is realized by a non-uniform electric field distribution in the piezoelectric plate. Monomorphs can exhibit strains up to 15% at 1 KV/mm but cannot sustain loads greater than approximately one pound.
Although the above indirect bending mode actuators can exhibit larger strains than direct mode actuators, the indirect mode actuators have very small load bearing capacities. In addition, the load bearing capacity of indirect mode actuators generally decreases as the achievable strain increases. Thus, there is a need to develop a piezoelectric or electrostrictive device that can produce large strains and sustain moderate loads.
The piezoelectric or electrostrictive sheets or elements used in the above devices can be fabricated by a number of methods, two of the most widely used which are atmosphere sintering and hot pressing. The metallic electrodes used for each of the above devices are typically deposited on the ceramic element. However, by utilizing a reduction process, the electrodes can be formed by chemically reducing the entire flat ceramic element by heat treatment in a carbon monoxide reducing atmosphere to produce a flat ceramic element having integral electrodes on each major surface. For example, Haertling (1990) 4th Int'l SAMPLE Electronics Conference, pp. 699-711 discloses a method for chemically reducing two major surfaces of a lead lanthanum zirconate titanate (PLZT) wafer by carbon monoxide reduction at temperatures between 600.degree. C. and 1000.degree. C. This produces a flat PLZT wafer having integral conductive electrodes on each major surface. The center of the element is a dielectric, and the PLZT wafer does not possess any asymmetrical internal compressive stress, nor can it produce any out-of-plane displacement.
U.S. Pat. Nos. 5,091,820 and 4,987,515 disclose a cylindrical ceramic core such as lead zirconate titanate (PZT) (PZT is a registered trademark of Vernitron, Inc.) having an integral electrode formed on each opposing end face of the core. The electrodes are formed by reducing the entire core in an atmosphere of hydrogen or nitrogen at a temperature between 650.degree. C. to 1000.degree. C. This produces a reduced metallic layer over the entire core. The reduction layer on the column surface of the core is removed leaving the two reduced electrodes on opposing end faces of the core. However, since the entire ceramic core is reduced, this process does not produce an internal stress bias nor does lit alter the original shape of the core.
U.S. Pat. No. 3,676,322 discloses a bimorph bender which includes a corrugated conductive center vane sandwiched between a pair of concave shaped ceramic wafers. Each wafer has conductive electrodes affixed to both sides thereof. The center vane is constructed to have its greatest stiffness at the center of the bimorph and its stiffness continuously decreases to where it is least at the circumference of the bimorph. As a result, the composite stiffness of the bimorph tends to overcome mechanical hysteresis because the center vane provides a restoring force which tends to overcome the natural resilience of the material and return the bimorph to its initial orientation as the driving signal goes through its zero crossings. In the fabrication method, after a flat circular ceramic wafer is made, the wafers are mechanically stressed into the concave configuration so that they can enclose the center vane. Next, the electrodes are deposited on each major surface of the ceramic element. Thus, the ceramic wafers do not have an integral electrode, do not possess any asymmetrical internal stress, the fabrication process is a complicated one and a corrugated conductive center vane is required to have less mechanical hysteresis than prior art flat wafers with a center vane of uniform thickness.
U.S. Pat. No. 3,447,217 discloses reduction/reoxidation methods for fabricating piezoelectric vibrators. One method comprises exposing a PZT sheet to a stream of hydrogen gas at a temperature of 800.degree. C. for 15 minutes to reduce the entire sheet. Next, the sheet is oxidized for 2 hours at 650.degree. C. in a stream of oxygen gas. This method produces an integral flat-shaped piezoelectric vibrator which comprises a reduced layer sandwiched between a pair of oxide layers. In another embodiment two flat elements are reduced and then one surface of each element is protected from reoxidation so that only one surface of the element is oxidized. This produces a flat element wherein one surface is reduced and the other is oxidized. As a result, these processes do not produce an asymmetrical internally stress biased concave shaped piezoelectric element.
Thus, there is a need to develop a piezoelectric or electrostrictive device that can produce relatively large strains and sustain moderate loads. There is also a need to develop such a device with an asymmetrical internal stress bias to provide above-plane axial displacement and increase mechanical strength. In addition, there is a need for a simplified fabrication process to produce such electrically active ceramic devices.